The plant genome's methylation status and response to stress: implications for plant improvement.

Plant improvement depends on generating phenotypic variation and selecting for characteristics that are heritable. Classical genetics and early molecular genetics studies on single genes showed that differences in chromatin structure, especially cytosine methylation, can contribute to heritable phenotypic variation. Recent molecular genetic and genomic studies have revealed a new importance of cytosine methylation for gene regulation and have identified RNA interference (RNAi)-related proteins that are necessary for methylation. Methylation differences among plants can be caused by cis- or trans-acting DNA polymorphisms or by epigenetic phenomena. Although regulatory proteins might be important in creating this variation, recent examples highlight the central role of transposable elements and DNA repeats in generating both genetic and epigenetic methylation polymorphisms. The plant genome's response to environmental and genetic stress generates both novel genetic and epigenetic methylation polymorphisms. Novel, stress-induced genotypes may contribute to phenotypic diversity and plant improvement.